a. Field of the Invention
The present invention relates to a method and apparatus for producing a hologram, in particular a dot matrix reflection volume hologram.
b. Related Art
Holograms are used widely as security features. Credit cards normally have an embossed hologram laminated in the credit card structure, for example on the surface by a hot-foiling technique. An embossed hologram is in fact a thin transmission hologram with reflective metallization. On the other hand, transmission holograms recorded in silver halide film are often referred to as “thick” or “volume” transmission holograms. Embossed holograms are relatively inexpensive to mass produce, and provide a bright, visible three-dimensional image when viewed by eye under good lighting conditions at the correct viewing angle. A limitation of such holograms is that diffraction effects cause the colouration of the holographic image to vary sharply with viewing angle, which is undesirable for some images, for which fixed or nearly fixed colours would be more appropriate, for example the image of a user's face or the image of a trademark, for which colour is an important feature.
In recent years, it has also become easier for criminals to make convincing copies of embossed holograms, partly because the hologram is encoded in a surface relief pattern on a reflective substrate from which it is possible to make a copy by illuminating the hologram with a laser, so as to transfer the image into a holographic recording material, which could later be used to produce multiple unauthorized copies.
In some cases, it is also possible to mimic the visible colouration effects of an embossed hologram by using suitable inks which present different colours across a range of viewing angles. Under casual inspection, a forged security device using colour variable inks may pass for a genuine embossed hologram security device.
In a further counterfeiting technique, simple diffractive hot foils may be used in multiple passes of a hot foiling system to simulate the multi-angle diffractive effects of a complex hologram.
It is an object of the present invention to address these issues.